1. Field of the Invention
The present invention relates to a gm-C tuning circuit and method for using same, and in particular to a gm-C tuning circuit using a poly-phase filter.
2. Background of the Related Art
Generally, a post-fabrication transconductance of an operational transconductance amplifier (OTA) should be adjusted according to the process-variations of components to maintain a selected precise cut-off frequency. A gm-C tuning scheme generally uses a master-slave tuning scheme to adjust a filter frequency that is inversely proportional to gm/C time constant (or RC time constant). In the master-slave tuning scheme, a master circuit is a copy of a slave block. The master circuit receives absolute frequency information from an external oscillator and adjusts transconductance to get the selected filter cut-off frequency. The control voltage of a master tuning feedback loop in the master circuit is copied to the slave block to reproduce the adjusted transconductance on the slave block. Then, the slave block becomes a main filter body whose cut-off frequency is controlled by the master circuit. Three related art master-slave tuning circuits will now be described.
FIG. 1 is a diagram that illustrates a master-slave voltage controlled oscillator (VCO) based tuning scheme. As shown in FIG. 1, a master block 110 outputs a control voltage 130 to a slave filter 140. As shown in FIG. 1, the master circuit 110 includes a comparator 111, a low pass filter 112, a rectifier 113, a voltage-to-current (V-I) converter 115, a low pass filter 116 that outputs a feedback signal 118 and a master VCO 120. The slave filter 130 is a copy of a master circuit.
In the master circuit 110, a comparator 111 compares a reference frequency Fref with an output frequency of the master VCO 120 fm. The low pass filter 112 low pass filters an output of the comparator to provide the control voltage 130 of the master circuit 110 that is copied to the slave filter 130. The rectifier 113 rectifies the output frequency fm from the master VCO 120, and the V-I converter 115 converts an input voltage from the rectifier 113 and a reference voltage Vref The low pass filter 116 receives a converted current from the V-I converter 115 and outputs the feedback signal 118 to the master VCO 120. In FIG. 1, an oscillation frequency of the master VCO 120 is determined by a time constant of each integrator, that is, the C/gm time constant when the gm-C integrator is used for the master VCO 120. The gm-C VCO based tuning circuit has relatively small hardware requirements and a simple feedback structure. However, the related art gm-C VCO tuning circuit has the disadvantage that a very high Q-factor is required for the VCO oscillation.
FIG. 2 is a diagram that illustrates another related art master-slave voltage controlled filter tuning scheme. As shown in FIG. 2, a master block 210 includes a comparator 211, a low pass filter 212, first and second rectifiers 213, 214, a voltage to current (V-I) converter 215, a low pass filter 216 and a master biquad 220. The master block 210 copies a control voltage 230 to a slave filter 240. The master circuit 210 is a copy of a slave filter 240.
As shown in FIG. 2, the comparator 211 of the master block 210 receives a reference voltage Fref frequency and an output frequency fm from the master biquad. The low pass filter 212 receives an output from the comparator 211 and outputs the control voltage 230 to the slave filter 240 and the master biquad 220. The first rectifier 213 receives the output frequency fm from the master biquad 220, and the second rectifier 214 receives the reference frequency Fref. The V-I converter 215 receives an output from the first rectifier 213 and the second rectifier 214, respectively. The low pass filter 216 receives an output from the V-I converter 215 and provides a second feedback signal to the master biquad 220. In the related art master-slave voltage controlled filter tuning circuit shown in FIG. 2, the quality factor of the filter is used for the feedback loop control signal 218. However, to provide sufficient sensitivity for phase tuning, the quality factor of the master circuit 210 must be large enough to provide sufficient sensitivity to phase tuning. The large quality factor of the master circuit 210 results in poor matching between the master block 210 and the slave block 240, which determines accuracy of the master-slave tuning system. The quality factor, Q, of the filter is used for the feedback loop control signal 218 is shown at equations 1A, 1B, 2A and 2B as follows:                                           H            ⁡                          (              s              )                                ⁢          L          ⁢                      xe2x80x83                    ⁢          P          ⁢                      xe2x80x83                    ⁢          F                =                              ϖ                          0              2                                                          s              2                        +                                          (                                                      ϖ                    0                                    /                  Q                                )                            ⁢              s                        +                          ϖ                              0                2                                                                        (1A)                                                      H            ⁡                          (              s              )                                ⁢          B          ⁢                      xe2x80x83                    ⁢          P          ⁢                      xe2x80x83                    ⁢          F                =                              ϖ                          0              s                                                          s              2                        +                                          (                                                      ϖ                    0                                    /                  Q                                )                            ⁢              s                        +                          ϖ                              0                2                                                                        (1B)            
wherein HLPH of Equation 1A and HBPF of Equation 1B are the Laplace transforms of the low pass filter and the band pass filter, respectively, of FIG. 2. Substituting the j{tilde over (xcfx89)} for the Laplace variable s yields.                                           H            ⁡                          (                              jϖ                0                            )                                ⁢          L          ⁢                      xe2x80x83                    ⁢          P          ⁢                      xe2x80x83                    ⁢          F                =                                            ϖ                              0                2                                                                    -                                  ϖ                                      0                    2                                                              +                                                (                                                            ϖ                      0                                        /                    Q                                    )                                ⁢                                  jϖ                  0                                            +                              ϖ                                  0                  2                                                              =                                    -              j                        ⁢                          xe2x80x83                        ⁢            Q                                              (2A)                                                      H            ⁡                          (                              jϖ                0                            )                                ⁢          B          ⁢                      xe2x80x83                    ⁢          P          ⁢                      xe2x80x83                    ⁢          F                =                                            jϖ                              0                2                                                                    -                                  ϖ                                      0                    2                                                              +                                                (                                                            ϖ                      0                                        /                    Q                                    )                                ⁢                                  jϖ                  0                                            +                              ϖ                                  0                  2                                                              =                      xe2x80x83                    ⁢          Q                                    (2B)            
FIG. 3 is a diagram that illustrates a related master-slave single integrator tuning scheme. As shown in FIG. 3, a master block 310 copies control voltage 330 to a slave filter 340. The master block 310 is a copy of the slave filter 330. As shown in FIG. 3, the master block 310 includes a first rectifier 313, a second rectifier 314, a voltage to current (V-I) converter 315, a low pass filter 316 and a single integrator 320. As shown in FIG. 3, the first rectifier 313 receives an output frequency fm from the single integrator 320, and the second rectifier 314 receives a reference frequency Fref. The V-I converter 315 receives output signals from the first rectifier 313 and the second rectifier 314. The low pass filter 316 receives output from the V-I converter 315 to output the control voltage 330 to the slave filter 340 and as a feedback signal 318 to the single integrator 320.
As shown in FIG. 3, the related art master-slave single integrator tuning scheme uses gm-C integrator 320 as the master of tuning to overcome various problems associated with the VCO type tuning scheme and the VCF type tuning scheme described above. In the single integrator tuning scheme shown in FIG. 3, the gm-C integrator 320 operates as a capacitor equivalent. The amplitude of the gm-C integrator 320 output and that of the input Fref are compared using the rectifier 313, 314 and the V-I converter 315. However, the input of the gm-C integrator 320 comes from an external oscillator and an output comes from an Operational Transconductance Amplifier (OTA) cell, which causes inaccurate tuning results.
In all of the above-discussed related art approaches, either a high Q factor results in poor matching between the master and the slave, or the input of gm-C integrator comes from an external oscillator and the output comes from the OTA cell which produces inaccurate timing results.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.
An object of the invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described hereinafter.
Another object of the present invention is to provide a master-slave circuit not limited by frequency or Q-factor requirements.
Another object of the present invention is to provide a master-slave tuning circuit using a poly-phase filter.
Another object of the present invention is to provide a master-slave gm-C poly-phase filter having the same electrical characteristics for a first filter and a second filter compared in the master-slave filters.
Another object of the present invention is to provide a gm-C poly-phase filter having output signals from high and low pass filters provided by the same circuit.
Another object of the present invention is to provide a master-slave tuning circuit having increased accuracy.
Another object of the present invention is to provide a more robust master-slave tuning circuit with increased accuracy and a simplified configuration.
To achieve the above described objects in a whole or in parts and in accordance with the present invention, there is provided a tuning circuit that includes a slave filter block that receives a first control signal and outputs a second control signal, and a master filter block that receives first and second prescribed reference signals and outputs the first control signal to the slave filter block, wherein the master filter block includes a polyphase filter that receives the first control signal, a first rectifier coupled to the polyphase filter, a second rectifier coupled to the polyphase filter, and a converter coupled to the first and second rectifiers that outputs the first control signal.
To further achieve the above described objects in a whole or in parts and in accordance with the present invention, there is provided a tuning circuit that includes a slave filter block having a cut-off frequency that receives a first control signal and outputs a second control signal, and a master filter block comprising a polyphase filter that receives first and second prescribed reference signals and outputs the first control signal to the slave filter block, wherein the second prescribed reference signal has a frequency approximately equal to the cut-off frequency of the slave filter block.
To further achieve the above described objects in a whole or in parts and in accordance with the present invention, there is provided a tuning circuit that includes a slave filter block having a cut-off frequency that receives a first control signal and outputs a second control signal, and a master filter block that receives the first control signal, the master filter block comprising a high pass filter and a low pass filter, the master filter block adapted to receive first and second prescribed reference signals and output the first control signal to the slave filter block, wherein the second prescribed reference signal has a frequency approximately equal to the cut-off frequency of the slave filter block.
To further achieve the above described objects in a whole or in parts and in accordance with the present invention, there is provided a method for tuning a master-slave tuning circuit that includes slave filtering a first control signal to output a second control signal, and master filtering first and second reference signals to output the first control signal for the slave filtering, wherein the master filtering includes high pass filtering the second reference signal using a first polyphase filter based on the first control signal, low pass filtering the second reference signal using a second polyphase filter based on the first control signal, rectifying at least one high pass filtered signal and at least one low pass filtered signal, wherein the second prescribed reference signal has a frequency approximately equal to a cut-off frequency of the slave filtering, and converting rectified high and low pass filtered signals to output the first control signal.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.